For single-carrier Frequency-Division Duplexing (FDD) systems like the High-Speed Packet Access (HSPA) FDD and Long-Term Evolution (LTE) FDD systems developed by the 3rd-Generation Partnership Project (3GPP), a major challenge to radio transceiver designers is the leakage of transmit (TX) signals into the receiver inputs, since the transmitters and receivers in FDD systems are (by definition) simultaneously active. This leakage primarily occurs due to the limited attenuation of duplex filters in the radio transceiver.
The leakage of the TX signal into the receiver may be viewed as containing two distinct parts, each of which can affect the receiver performance. First, the leakage includes TX noise, at frequencies extending far from the intended bandwidth of the TX signal, which can extend into the frequency range used by the signals to be received. This TX noise directly leads to co-channel interference. Second, the leakage includes an attenuated version of the actual modulated TX signal, which leads to a strong signal at the receiver input, albeit at a frequency different from the desired receiver signal. The receiver must be able to accommodate this out-of-band signal without becoming saturated.
Leakage problems become more complex in transceivers designed to support carrier aggregation, in which the transceiver supports the simultaneous use of two or more uplink and/or downlink carriers. Such transceivers can have two or more local oscillators, at different frequencies, operating simultaneously. The signals from these local oscillators can interact with one another, generating nonlinear intermodulation products that can create spurious frequency components in LO signals. This is particularly harmful for the receivers, in which the spurious frequency components of the LO may lead to undesired frequency-conversion of the leaked transmitter signals that will interfere with the desired signals to be received.
While various techniques have been proposed for combatting transmitter leakage in simple transceiver designs, improved techniques are needed to address the problems created in transceivers that include two or more simultaneously operating receivers.